It is well known that amines such as hexamethylene diamine, propyl amines, butyl amines, benzyl amines, tallow amines, ethyl amines, etc., may be produced by the catalytic hydrogenation of nitriles such as proprionitrile, butyronitriles, tallow nitriles, acetonitriles, etc., in the presence of catalysts and other substances such as ammonia and/or caustic alkali. As set forth in U.S. Pat. No. 3,821,305, the entire subject matter of which is incorporated herein by reference, one such process is described in which hydrogenation is conducted in liquid phase at pressures of from 20-50 atmospheres and temperatures of 60.degree.-100.degree. C. in the presence of finely divided Raney catalyst and an inorganic base. Hydrogen and adiponitrile are fed into a liquid reaction medium consisting of hexamethylenediamine, water, the inorganic base, and the catalyst, in which medium the content of base is maintained in the range of 0.2-12 moles per kilogram of catalyst, while the content of water is maintained in the range of 2-130 moles per mole of the base.
In typical continuous processes utilizing a Raney nickel or Raney cobalt hydrogenation catalyst, the rate at which the catalyst is fed into the reaction medium must be carefully controlled. Active catalysts of that type are pyrophoric, however, and are therefore normally kept out of contact with air by transporting and storing the catalyst in a relatively inert liquid. Hence, in some of the aforementioned processes, the rate at which the catalyst is fed into the reaction medium is desirably controlled by suspending the catalyst in such a liquid so as to disperse the catalyst substantially uniformly through the liquid in a known concentration of catalyst per unit volume of the suspension, and then controlling the volumetric flow rate of the suspension into the reaction mixture. Examples of processes in which the catalyst feed rate may be conveniently controlled in this way are described in U.S. Pat. No. 3,821,305, the disclosure of which is incorporated herein by reference, and in U.S. Pat. No. 3,056,837, the disclosure of which also is incorporated herein by reference.
However, Raney nickel and cobalt catalysts in such processes have been plagued by high deactivation rates under certain conditions when utilized in the hydrogenation of nitrites. For example, an article in Chemical Engineering Science, Vol. 47, No. 9-11, 2289-94 (1992), indicates that nitrites deactivate nickel or cobalt catalysts, such as Raney nickel catalysts. More recently, efforts have been made to reduce such catalyst deactivation rates. For example, it is also known in such low pressure hydrogenation systems to utilize high liquid recirculation velocities in an attempt to provide good mixing conditions found in turbulent flow so as to enhance catalyst stability and increase mass transfer coefficients as set forth in Chemical Engineering Science, Vol. 35, 135-141 (1980).
Additionally, efforts have been made to study reactors to determine the effect of operating conditions on catalyst deactivation rates. For example, in Catalysis Today, 24, 103-109 (1995) catalyst deactivation effects under various operating conditions for hydrogenation of adiponitrile, in a continuous bench scale slurry bubble column reactor were investigated. The reactor was considered to be mixed perfectly because the temperatures at the top and bottom of the column were identical and the differences in concentration between the samples taken at the top and the bottom of the column were less than 15%.
Efforts have also been made to reduce catalyst deactivation by physically blocking the active catalyst sites or access to the sites and equipment fouling in hydrogenation reactions by increasing mass transfer rates in the reactor system, i.e., see "Pumped-up Mixer Improves Hydrogenation," Chemical Engineering, June 1998, p. 19, in which increased mass transfer rates reduced catalyst physical deactivation and equipment fouling in hydrogenation reactions for the production of edible oils. However, the local bulk concentrations of reactants in such reactors varies considerably and will not inhibit chemical catalyst deactivation (i.e., catalyst deactivation by irreversibly depleting the catalyst of elements (e.g., interstitial hydrogen)) necessary for adequate catalyst activity in nitrile hydrogenation reactions.
However, it has now been discovered that contrary to the assumptions and inferences made in the above-mentioned prior reactor configurations for the hydrogenation of nitrites, the reactants in such reactors are not perfectly mixed across the diameter of the reactor. According to the present invention, studies have been made that indicate the local bulk nitrile concentration in such reactors is not uniform and through most of the reactor the local bulk nitrile concentration exceeds that stoichiometrically required to completely deplete the local bulk hydrogen concentration, which leads to an increased catalyst chemical deactivation rate. Accordingly, there is a need to provide certain reactor conditions that would provide reduced chemical catalyst deactivation rates in nitrile hydrogenation systems.